Baby Website Rick
Analysis
All the references to pickles implies it's an insecure deserialization challenge. pickle
is a serialization format used in python.
If we check the cookies, we get the following:
Our guess is that this is a pickled python object, and decoding the base64 seems to imply that to us too:
Unpickling
Let's immediately try to unpickle the data, which should give us a feel for how data is parsed:
The error is quite clear - there's no anti_pickle_serum
variable. Let's add one in and try again.
That error is fixed, but there's another one:
Here it's throwing an error because X (anti_pickle_serum
) is not a type object - so let's make it a class extending from object
!
And now there's no error, and we get a response!
So the cookie is the pickled form of a dictionary with the key serum
and the value of an anti_pickle_serum
class! Awesome.
Exploitation
For an introduction to pickle exploitation, I highly recommend this blog post. Essentially, the __reduce__
dunder method tells pickle how to deserialize, and to do so it takes a function and a list of parameters. We can set the function to os.system
and the parameters to the code to execute!
Here we create the malicious class, then serialize it as part of the dictionary as we saw before.
Huh, that looks nothing like the original cookie value (which starts with KGRwMApTJ3
)... maybe we missed something with the dumps
?
Checking out the dumps()
documentation, there is a protocol
parameter! If we read a bit deeper, this can take a value from 0
to 5
. If we play around, protocol=0
looks similar to the original cookie:
Let's change the cookie to this (without the b''
):
As you can see now, the value 0
was returned. This is the return value of os.system
! Now we simply need to find a function that returns the result, and we'll use subprocess.check_output
for that.
For reasons unknown to me, python3 pickles this differently to python2 and doesn't work. I'll therefore be using python2 from now on, but if anybody know why that would happen, please let me know!
Now run it
And input it as the cookie.
As can now see that there is a flag_wIp1b
file, so we can just read it!
While it's tempting to do
subprocess.check_output
requires a list of parameters (as we see here) and the filename is a separate item in the list, like so:
And boom - we get the flag!
HTB{g00d_j0b_m0rty...n0w_I_h4v3_to_g0_to_f4m1ly_th3r4py..}
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